Unless you really love each one, what is the point of memorizing all 48 perfectly? Come to think of it, why do people learn all 32 Beethoven Sonatas, all 24 Chopin Etudes, etc.? It seems silly to learn a piece that one doesn't really enjoy just for the sake of saying that one can play all in the set. For example, the only Chopin Etudes I ever intend to learn would be #3, 4, 6, 10, and 12 of op. 10; and #3, 4, 5, 10, and 12 of op. 25. Those are the etudes I like, and I don't see the purpose behind putting effort into something like op. 10 #2 (please forgive me if that is somebody's favorite, I am just saying that I don't like it and don't plan on learning it).